1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, and more specifically to a control system for controlling an EGR (exhaust gas recirculation) system of an engine of a type having a stratified charge combustion mode and a homogeneous charge combustion mode.
2. Description of Related Art
Recently, the technique of in-cylinder direct fuel injection in a spark ignition engine such as gasoline engine is under development to improve the fuel efficiency and emission performance by using stratified charge combustion and homogeneous charge combustion.
In a low and medium load region, a control system for such a type operates an engine in a stratified combustion mode by injecting fuel directly into a combustion chamber during the compression stroke so as to produce a stratified combustible mixture only around the spark plug. The thus-achieved stratified combustion enables stable combustion with an ultra lean mixture, and hence significant improvement in the fuel efficiency and emission performance of the engine.
In a high load region over a predetermined engine load, the engine is operated in a homogeneous combustion mode to meet a demand for higher output torque. In the homogeneous combustion mode, fuel is injected during the intake stroke so as to produce a homogeneous air fuel mixture. (In some examples, a fuel injection valve is provided separately in an intake port.)
The control system changes over the combustion control mode between the stratified combustion mode and the homogeneous combustion in accordance with one or more engine operating conditions.
To reduce NOx emission, an engine for a vehicle is generally equipped with an EGR system for recirculating part of the exhaust gases from the exhaust system to the intake system. A desired target EGR rate is not equal between stratified combustion and homogeneous combustion. Therefore, there is a need for switching the target EGR rate in accordance with a changeover of the combustion.
Japanese Patent Provisional (Kokai) Publication No. 7(1995)-269416 shows an EGR control system for an engine having stratified combustion and homogeneous combustion. In the stratified combustion, this system determines the target EGR rate in accordance with the engine speed and engine load. When the combustion is changed to the homogeneous combustion, this control system reduces the target EGR rate to zero and thereby cut off EGR by closing an EGR valve.